Generally, as a means for improving the mechanical properties of rolled steel, refinement of the structure of rolled steel is well known. Improvement of the mechanical properties of rolled steel provides many advantages such as realization of lightweight of a steel structure. Many methods for manufacturing steel having a micro-structure, that is, fine-particle steel have been proposed and as typical methods, (1) the high-pressure rolling method and (2) the control rolling method may be cited.
Among them, (1) the high-pressure rolling method is described in Japanese Patent Laid-Open Publication No. 123823/1983 and Japanese Patent Publication 65564/1993. Namely, the method applies high pressure to austenite particles, thereby promotes the straining transformation from the austenite (γ) phase to the ferrite (α) phase, and refines the structure.
Further, (2) the control rolling method is a method for realizing refinement of ferrite particles containing components of Nb (niobium) and Ti (titanium) which can be easily increased in tension by the deposition increasing operation of Nb and Ti and also promotes the straining transformation from they phase to the a phase when the cold rolling (ferrite region rolling) is executed by the recrystallization suppression operation for austenite particles of Nb and Ti.
The control rolling method executes the finishing rolling in the low temperature zone (800° C. or less), so that it has a disadvantage that the deformation resistance of steel to be rolled is extremely high, thus the load on the strip rolling apparatus is large. On the other hand, the high-pressure rolling method, as indicated in Japanese Patent Publication 65564/1993 aforementioned, cannot be executed industrially by a general hot strip mill and requires use of a special rolling apparatus. The reason is that, as described in the aforementioned patent publications, continuous rolling at a high pressurization rate (for example, 40% or more) which cannot be realized by a general rolling apparatus is required.
When fine-particle steel is to be manufactured industrially and commercially by executing the high-pressure rolling method, in addition to that a rolling apparatus of a general hot strip mill type cannot be used, the following problems are imposed.
i) Owing to execution of rolling under high pressure, that is, at a high pressurization rate, faults due to the rolling load may be often caused. Namely, there is a case that the rolling load reaches the intrinsic limit value (mill power restriction and machine strength) of the rolling apparatus and rolling becomes impossible. Furthermore, for steel to be rolled, a predetermined pressurization rate cannot be realized and large edge drops are caused. The reason that the predetermined pressurization rate cannot be obtained is that particularly when the plate thickness on the exit side of the rolling apparatus is 2 mm or less and the pressurization rate is 40% or more, the rolling load is large and the deformation resistance is high, so that the rolling flatness is increased. In this case, even if the pressure is increased so as to execute rolling under high pressure, the pressurization rate is not increased. The reason for increasing the edge drop is that a high load is applied to the neighborhood of the edge (the end in the width direction) of steel to be rolled and no good plate profile can be obtained.
ii) Difficulty in keeping the temperature of steel to be rolled is also a serious problem. The reason is that when rolling is executed at a high pressurization rate using a mill of a plurality of stands, the temperature of steel to be rolled is increased remarkably due to working heat generation and it is not easy to keep it at the temperature (the range from the transformation point of Ar3 to Ar3+50° C.) suited to execution of the high-pressure rolling method. When steel to be rolled is accelerated and the feed speed is increased, the strain speed is increased and the working heat generation is increased, so that it becomes difficult more and more to keep the temperature.
iii) Faults relating to the thermal load of the rolls are often caused. When rolling at a high load providing a high pressurization rate is executed, the working heat generation of steel to be rolled is also increased and the thermal load of the rolls is increased in correspondence to it. As a result, a thermal crown that each roll is extended in diameter at the center thereof is easily generated. The thermal crown may not be eliminated only by cooling each roll depending on the degree thereof, and steel to be rolled gets worse in the shape, and a stable flow of plate may not be obtained easily.
iv) The rolls are worn out strongly and the shape (crown) of steel to be rolled easily gets worse. The reason is that during rolling at a high pressurization rate and a high load, the thermal or dynamic load applied on the rolls is high, so that the wear of the rolls easily progresses. At the part of each roll in contact with the edge of steel to be rolled, the rolling load is high, so that the wear easily progresses and the profile of steel to be rolled which is important for the quality thereof is easily reduced greatly. Further, when the rolls are easily worn out, the cost for maintenance such as grinding or exchange of the rolls is increased.
Therefore, an object of the present invention is to solve the aforementioned problems concerning manufacture of hot rolled steel plates of fine-particle steel by providing a hot rolling apparatus for enabling smooth manufacture of those steel plates and a fine-particle steel manufacturing method.
Further, another object of the present invention is to provide a continuous hot rolling method suited to manufacture of hot rolled steel plates of fine-particle steel which is superior in respect of cost to effect.
Further, still another object of the present invention is to provide a continuous hot rolling method for smooth manufacture of thick plates using a hot rolling apparatus capable of manufacturing thin plates.